The present invention relates to a beading apparatus for making an electron gun assembly, and particularly to a beading apparatus for making an electron gun assembly having insulating support rods with a plurality of indexing cavities formed therein.
The electrostatic lens elements of an electron gun assembly are serially arranged to accelerate and focus at least one electron beam along a generally longitudinally-extending electron beam path. The lens elements of the gun assembly are mechanically secured to at least a pair of generally longitudinally-extending insulating support rods by means of support tabs extending from the lens elements and embedded into the support rods.
The support tabs may be integral with the lens element or the support tabs may be attached, for example by welding, to the body of the lens elements. In either case, the portions of the support tabs embedded into the support rods include shaped projections or claws formed into the end of the support tabs to firmly anchor the tabs within the support rods. Attachment of the tabs to the support rods is accomplished in an operation called beading. One example of a beading apparatus is shown in U.S. Pat. No. 3,527,588 issued to Aiken et al., on Sept. 8, 1970. Occasionally, during the beading operation, one or more of the support rods became misaligned resulting in improper spacing between lens elements or resulting in incomplete coverage of the claw of the support tab by the insulating support rod. Either condition is undesirable and causes distortion of the electrostatic fields within the electron gun assembly which perturb the electron beam.
An early attempt to improve support rod alignment by reducing the lateral movement of the support rod is shown in U.S. Pat. No. 3,609,400 issued to Marks et al., on Sept. 28, 1971. In the Marks et al. structure, shown in FIGS. 1 and 1a of the present application, a beading block A includes a beading trough B in which the insulating support rod C is nested. The accuracy of the support rod alignment in the Marks et al. patent depends on the accuracy with which the width of the support rod can be controlled. The present industrial width tolerance for pressed multiform support rods up to 49 mm (millimeters) in length is .+-.0.254 mm. The arrows in FIGS. 1a and 1b serve to indicate the direction of motion of the beading block A during the beading operation.
A similar beading apparatus utilized to fabricate an electron gun structure of a pickup tube is shown in U.S. Pat. No. 4,169,239 issued to Ehata et al., on Sept. 25, 1979. The Ehata et al. structure is reproduced in FIGS. 2 and 3 of the present application. In Ehata et al., the insulating support rods E are supported on beading bases D which are rotated toward the stacked lens elements. As the Ehata et al. patent discloses, if the viscosity of the fused glass support rod is low, the accuracy with which the electrodes are assembled is decreased due to thermal and mechanical shock created at the time the molten supporting rod contacts the lens element support tabs.
It is also known in the art that a secure but somewhat random placement of the insulating support rod on the beading base can be accomplished by providing the beading base with a vacuum holding capability. In practice, when the support rod is held in the vertical position with a vacuum transducer, the vacuum retention force can be removed when the beading fires are turned on in order to eliminate gaseous combustion contamination. It has been found that the retaining force of the impinging gas flame is great enough to retain the support rod stationary in the vertical position. However, because of interrelated width tolerances between the support rod and the beading base, the support rod can be offset in a lateral direction during initial placement thereby resulting in a misalignment of the support rod.
In a recently developed electron gun assembly having at least two self-indexing support rods, each of the self-indexing support rods includes at least two indexing cavities formed in a surface of the support rods to align the rods during the beading operation. The indexing cavities permit the support rods to be beaded to the claws of the lens element support tabs without regard to the width tolerance of the support rods.